The letterpress printing is a kind of relief printing to develop with water or a mixture of water and an alcohol. As well known, letterpress printing is water developed, and is excellent in economic properties, versatility and causing no environmental contamination. For this reasons, the flexographic printing is reevaluated, and recently has attracted attention due to an improvement in printing accuracy.
In Japan, as printing methods, offset printing and gravure printing are relatively widely used, while relief printing such as letterpress printing and flexographic printing has not been comparatively used so often. However, the letterpress printing has the following advantages over the other printing methods:    (1) An anastatic printing plate is used, therefore the printing on a printing object which is not flat or smooth such as a corrugated fiberboard is possible. In addition, the printing can be achieved at relatively high speed and low cost.    (2) Clear printing can be conducted inexpensively on an object having a small area such as a label.    (3) Printing can be made at a high ink density as compared to offset printing, thus enabling clearer printing.    (4) The profile of characters or images is sharp as compared to that obtained by gravure printing.    (5) Water development results in lower cost and lower risk of environmental contamination than solvent development.    (6) The time required for stabilizing the color from the start of the printing is short, and hence the waste of a printing medium such as paper can be reduced, which is advantageous from an economical point of view.    (7) The printing can be achieved in small lots and many kinds.
In this way, the relief printing is a conventional printing technique, and the letterpress printing is also a printing technique conventionally used and has been well known to have the various advantages mentioned above. However, a frequency of the use of the flexographic printing relative to the whole printed matter has not been high.
However, in recent years, as the materials used for printing ink and printing original plate are improved in quality and interests in the environmental issues are increasing, the relief printing including letterpress printing is reevaluated, and studies and developments are being vigorously made with a view to positively using such printing.
Here, a progress in development of the printing original plate itself is as follows. The letterpress printing plate has been formed from a metal or a hard resin for a long period, and produced by engraving the hard layer to form a negative image of characters or images to be printed. However, a photosensitive resin is begun to be used recently. The photosensitive resin generally includes an elastomeric binder, at least one monomer, and a photopolymerization initiator. An original printing plate produced using this photosensitive resin is a plate member provided with a layer of the photosensitive resin on a support.
In production of a letterpress plate using this original printing plate, first a film (mask) having a negative pattern of an image such as a text and a picture to be printed is placed on the photosensitive resin layer of this original printing plate, and the photosensitive resin layer is then exposed to a chemical ray via this mask. The portion irradiated with actinic radiation undergoes a photopolymerization reaction and then is cured. Subsequently, the uncured portion is washed away with a developing solution, so that a relief pattern corresponding to the image remains, thus forming a letterpress printing plate. In the letterpress printing, an ink is put on the top surface of the relief pattern, which is compressed on a printing medium such as paper to carry out printing.
In original printing plate using this photosensitive resin, it has been desired that the following problems that had been pointed out be solved.    (i) When a final pattern of a negative mask is required to be modified, it is necessary to remake the whole photographic negative mask since it is impossible to modify the mask partly, so that it takes comparatively lots of time for modifying the mask.    (ii) A size of the negative mask may easily alter due to changes of temperature and humidity because the negative mask is composed of a negative film. Thus, even if the same negative mask is used, each pattern-forming step including the light exposure and development of the photosensitive resin layer may result in different accuracy of the printing plate products, due to difference in timing and environment for performing the procedure.    (iii) In the pattern forming step, a substance such as dust which inhibits light incidence tends to come in between the negative mask and the photosensitive resin layer. If such a substance has come in, a pattern image obtained after the exposure and the development may be disturbed, which may deteriorate printing quality of the printing plate.
In order to solve such problems, several kinds of original printing plates with new constitution were developed recently (Patent Documents 1 and 2). The constitutional feature that is common over the original printing plate of these days is in that an infrared ray-sensitive material is at least formed on the photosensitive resin layer, in which the infrared ray-sensitive material layer is constituted to be capable of functioning as the conventional photographic negative mask. This infrared ray-sensitive material layer is nontransparent to the actinic rays light curing the photosensitive resin, and sensitive to infrared rays. The term “infrared ray-sensitive” means to be evaporated and/or decomposed by exposure using infrared laser beams, so-called ablation. Thus, the infrared ray-sensitive layer is also referred to as the infrared ablation layer.
By laminating the infrared ablation layer, printed image information can be recorded directly on original printing plate for letterpress printing by using infrared laser beams, whereby a mask film consisting of a conventional negative film can be omitted. The printed image information can be created, saved, modified, and output as a digital information. When the letterpress printing plate is made by the constitution providing the infrared ablation layer, the cost for image information processing required in use of a conventional mask film is sufficiently reduced.
Several compositions of the infrared ablation layer have been suggested; however, they are substantially similar. For example, in the original printing plate as described in Patent Document 1, the infrared ablation layer is constituted with a water-soluble binder, a water-dispersible binder, a binder soluble or dispersible in a mixture of water and an alcohol, and an infrared absorption material which is finely-dispersed in these binders, and exhibits strong absorbance in the wavelength range of 750 to 20000 nm, particularly 750 to 5000 nm. In addition, in order to disable transmission of chemical rays as needed, all compounds that absorb ultraviolet rays can be used.
The infrared absorbent material and all compounds that absorb ultraviolet rays include a dye and a pigment. Specific examples include phthalocyanine, substituted phthalocyanine derivatives, cyanine, merocyanine dyes, as well as polymethine dyes, carbon black, graphite, chromium oxide, iron oxide and the like as the pigment.
In Patent Document 2, the infrared ablation layer includes an IR absorbent metal layer.
Patent Document 1: Japanese Patent No. 3429634
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-101751